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[PATCH v2] target/arm: adjust program counter for wfi exception in AArch

From: Jeff Kubascik
Subject: [PATCH v2] target/arm: adjust program counter for wfi exception in AArch32
Date: Mon, 13 Jan 2020 10:59:34 -0500

The wfi instruction can be configured to be trapped by a higher exception
level, such as the EL2 hypervisor. When the instruction is trapped, the
program counter should contain the address of the wfi instruction that
caused the exception. The program counter is adjusted for this in the wfi op
helper function.

However, this correction is done to env->pc, which only applies to AArch64
mode. For AArch32, the program counter is stored in env->regs[15]. This
adds an if-else statement to modify the correct program counter location
based on the the current CPU mode.

Signed-off-by: Jeff Kubascik <address@hidden>

I am using the ARMv8 version of QEMU to run the Xen hypervisor with a guest
virtual machine compiled for AArch32/Thumb code. I have noticed that when
the AArch32 guest VM executes the wfi instruction, the hypervisor trap of
the wfi instruction sees the program counter contain the address of the
instruction following the wfi. This does not occur for an AARch64 guest VM;
in this case, the program counter contains the address of the wfi
instruction. I am confident the correct behavior in both cases is for the
program counter to contain the address of the wfi instruction, as this works
on actual hardware (Xilinx Zynq UltraScale+ MPSoC).

I have tested the above patch and it works for Xen with both an AArch64
guest (Linux) and an AArch32 guest (RTEMS). I'm still getting accustomed to
the QEMU code base, so it may not be correct. Any feedback would be greatly

Jeff Kubascik

Changes in v2:
- Added braces {} to if-else statement, per patchew feedback
 target/arm/op_helper.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/target/arm/op_helper.c b/target/arm/op_helper.c
index e5a346cb87..27d16ad9ad 100644
--- a/target/arm/op_helper.c
+++ b/target/arm/op_helper.c
@@ -295,7 +295,12 @@ void HELPER(wfi)(CPUARMState *env, uint32_t insn_len)
     if (target_el) {
-        env->pc -= insn_len;
+        if (env->aarch64) {
+            env->pc -= insn_len;
+        } else {
+            env->regs[15] -= insn_len;
+        }
         raise_exception(env, EXCP_UDEF, syn_wfx(1, 0xe, 0, insn_len == 2),

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